Amplification and/or hyperactivation of c-MYC (MYC) occurs in 20-40% of human malignancies and confers poor prognoses in many cancer subtypes. This well-studied oncogene is commonly known to drive oncogenesis, but direct targeting of MYC has been unsuccessful to date. Aberrant activation of MYC also provides unique stresses to cancer cells. Consequently, tumors become dependent on pathways that enable them to tolerate this stress. Such stress-support pathways represent ideal therapeutic targets because cancer cells become hyper-dependent on them for survival whereas normal cells and tissues do not. However, a major barrier to developing targeted therapies for MYC-driven breast cancer is the gap in understanding the stress support pathways necessary to tolerate aberrant MYC activation. Identifying these stress support networks whose inactivation kills MYC-driven cancers would have enormous impact on our understanding of the disease and how we treat afflicted patients. Using a genome-wide forward genetic screen, we searched for genes required to tolerate aberrant MYC activation (Kessler et al, Science 2012). We discovered that cells are strongly dependent on the spliceosomal protein BUD31 to tolerate MYC hyper-activation, and BUD31 inactivation impairs MYC- dependent breast cancers in vivo. Although BUD31 is poorly understood in humans, the Bud31 yeast homologue is a member of the core spliceosome, which removes pre-mRNA introns, and BUD31 is required for spliceosomal activation. Our mass spectrometry studies reveal that human BUD31also interacts with the core spliceosome and regulates splicing in human cells. This raises the provocative hypothesis that cells become hyper-dependent on the spliceosome to tolerate aberrant MYC activity. Notably, MYC-dependent breast cancer cells are highly sensitive to compound D1, a novel splicing inhibitor that inhibits BUD31- interacting components of the spliceosome, whereas MYC-independent cells are resistant to this agent. We hypothesize that hypomorphic perturbations of the spliceosome, including BUD31, lead to MYC-synthetic lethality. Herein, we propose to elucidate whether BUD31 interaction with the spliceosome is required in cells stressed by MYC hyperactivation, to discover how excess MYC activity confers dependency on the RNA splicing machinery, and to exploit these vulnerabilities as therapeutic targets in MYC-driven breast cancers. By understanding how BUD31 and its associated complex play a role in coping with MYC-stresses, we will identify potential targets for therapeutics that can be specifically applied to aggressive MYC-hyperactive breast cancers. Furthermore, insight into stress support networks may be applied to other MYC-driven malignancies, which will make a substantial impact on the treatment of cancer patients.